Description.

The present invention concerns a pouring tube for conveying a molten metal from an upper metallurgical vessel to a lower one. In particular, it concerns a pouring tube of refractory material for the transfer of molten steel from a distributor to an ingot mold or alternatively from a pouring ladle to a distributor.

The pouring tubes designed to convey the molten metal from one metallurgical vessel to another are wear pieces highly stressed to the point that their service life could limit the pouring time. Numerus devices for introducing and changing the tube described recently in the prior art permitted resolving this problem (see, for example, the European Patents 192019 and 441927). As soon as the erosion in the vicinity of the meniscus, and even sometimes inside of the pouring tube, reaches a certain level, the worn tube is replaced with a new tube within a sufficiently short time so as not to interrupt the pouring.

In these devices, a pouring tube comprised of a tubular piece equipped at its upper end with a plane plate designed to slide in guides against the lower plane face either of a pouring orifice such as a nozzle or of a fixed bottom plate coupled to a mechanism for regulating the pouring jet inserted between the pouring orifice (e. g., nozzle) and the pouring tube is generally used.

These tubes can be in one piece or can consist of an assembly of several refractory pieces.

In most cases, the side faces of the plate, the lower face of the plate and the upper end of the tubular part of the tube are protected by a metal case.

Despite the considerable advantages imparted to the state of the art by the systems described above and the continuous improvement that they have undergone in recent years, there are still some problems.

In particular, the appearance of cracks or microcracks is frequently observed in the pouring tube at the level of the junction between the tubular piece and the plate located at the upper end of the tubular piece. These cracks or microcracks can occur from the preheating of the tube prior to its use, the first moments of its use or during its use.

Even if these cracks have a negligible size in some cases, they must be taken into account. The passage of the molten metal in the tube induces indeed a substantial aspiration of ambient air. Atmospheric oxygen or even nitrogen are substantial sources of contamination of the molten metal, in particular, steel. In addition, under the combined action of oxygen and very high temperatures, the refractory material can be considerably degraded at the point of arrival of oxygen, i. e., the crack. This degradation further contributes to a local deterioration of the refractory material and

enlarges the crack to the point that it could be necessary to stop the pouring.

There are refractory materials more resistant to cracking. However, these materials are generally sensitive to other phenomena such as erosion, corrosion, etc.

Furthermore, the cracking can be caused by a thermal shock, a mechanical shock or the application of excessively high mechanical stresses such as pressure or vibrations.

In particular, in a device for introducing and changing the tube, the plate undergoes substantial bending stresses perpendicular to its surface that may be responsible for the formation of cracks at the upper end of the tubular part. It is indeed observed that the upper plate can be deformed by bending around an axis parallel to the direction of the guides in which the said plate slides. Thus, it is difficult to reach a compromise based only on the chemical composition of the refractory material that solves all these problems.

The applicant thus undertook to find a means of reducing the formation of cracks in the pouring tube without having to modify the composition of the refractory material.

European patent application 601,700 discloses already a pouring tube of refractory material comprised of a tubular element and equipped at its upper end with a plane plate. The side faces and lower faces of the said plate and the whole tubular part of this pouring tube are protected by a metal case. A reinforcing cone surrounds the upper end of the tubular part.

Such a reinforcing cone stiffens homogeneously the structure all around the upper end of the tubular part. However, as explained above, it has been observed that, in case of use in a device for introducing and changing a pouring tube, the pouring tube is subject to constraints which are not homogeneously distributed. Moreover, its large hindrance prevents the use of a cone reinforced tube such as disclosed in the European patent application 601,700 in a device for introducing and changing a pouring tube.

The present invention thus concerns a pouring tube of refractory material comprised of a tubular element and equipped at its upper end with a plane plate, the side faces and lower face of the said plate and the upper end of the tubular part being protected by a metallic case, said plate being reinforced in its lower face by mechanical means that increase its stiffness that is characterized in that these stiffening means are comprised of one or more U-shaped metallic pieces located on either side of the tube and attached fixedly to the part of the metal case protecting the lower face of the plate.

In this case, it proved advantageous to attach the end of the U-shaped pieces fixedly to the upper end of the tubular part.

Preferably, the attachment means of the U-shaped pieces comprises at least two linear welds arranged on either sides of and in contact with the upper end of the tubular part and parallel to the direction of the guides (sliding direction). These welds attach thus fixedly the ends of the U-shaped piece to the lower face of the plate tangentially to the

tubular part.

Advantageously, the gaps between the ends of the respective U-shaped elements are filled in with another weld which extends thus orthogonaly to the plate, forming a T- shaped weld with the linear weld attaching the U-shaped piece to the lower face of the plane plate.

In this case also, it is preferable that the U-shaped pieces permit at least a limitation of the bending of the plate around an axis parallel to the direction of the guides in which the said plate slides. To this end, the U-shaped pieces can be disposed so that their side branches are parallel to the direction of the guides.

The side branches of the U-shaped pieces advantageously have a length such that the ends of the two U-shaped pieces are close to each other and can even be in mutual contact. According to this form of implementation, the plate is rendered more rigid in two perpendicular directions. The U-shaped pieces are preferably arranged so that their side branches are parallel to the direction of the guides; in this manner, the bending of the plate around axes parallel and perpendicular to the direction of the guides in which the said plate slides is limite.

These pieces have the effect of increasing the height of the piece subjected to bending stresses and consequently increase its bending inertia.

These pieces have a height between 1 and 20 cm, preferably between 2 and 8 cm. The applicant was able to determine that the pouring tubes according to the present invention became cracked definitely less than conventional tubes.

In the case of a pouring tube for conveying a molten metal from a distributor to an ingot mold, the lower end of the pouring tube quite frequently has two lateral openings for the flow of molten metal into the ingot mold. These openings located on either side of the tube should be oriented in the ingot mold along an axis parallel to the larger dimension of the ingot mold. It is imperative that the pouring tube engages correctly in the ingot mold. A wrongly oriented tube would induce a flow of the molten metal toward the walls of the ingot mold and would be responsible for substantial perturbations in solidification zone of the metal. These perturbations can result in a laceration of the skin of the solidifying slab and thus a breakthrough of molten metal under the ingot mold. To avoid orienting a replacement pouring tube wrongly in a device for introducing and changing a pouring tube, it was proposed to provide the lower face of the plane plate with an anti-inversion device. This device gives the part of the plate designed to slide in the guides an asymmetric (rectangular or ellipsoidal) form so that it is impossible to engage the plate in the said guides with an unintended orientation. The U. S. Patent 5,188,743, for example, describes such a system.

Advantageously, the mechanical means that increase the stiffness of the plate also serve as anti-inversion device. For this purpose, they are arranged around the tubular part of the pouring tube in an asymmetric manner so that the tube cannot be engaged

with an incorrect orientation in a device for introducing and changing the tube.

During the development of the present invention, the applicant also found that in some cases the very high temperatures reached by the different parts of the pouring tube during the passage of the molten metal can cause a softening of the mechanical means reputed to increase the rigidity of the plate and strip them of any efficacy. This problem, when it was observed, could be resolved by using pieces having a heat- dissipating structure. For example, the lower end of the said pieces can be indented or crenellated to induce a substantial air circulation at their level. In a variant or in combination, at least a part of these pieces can also be provided with fins in order to induce a substantial air circulation at their level.